1. Field of the Invention
The present invention relates generally to an active matrix type liquid crystal display wherein switching elements serving as active elements are connected to a plurality of scanning lines and a plurality of picture signal lines perpendicular thereto.
2. Related Background Art
Liquid crystal displays of this type are widely used as graphic displays for use in information equipment terminals and thin type televisions. Particularly, in recent years, in order to increase the usual picture area on a transparent insulating substrate of the same area and in order to decrease producing costs, there has been developed a driving circuit including active matrix type liquid crystal display wherein a scanning line driving circuit and a picture signal line driving circuit are integrally formed on a transparent insulating substrate similar to picture element thin-film transistors.
The scanning line driving circuit and picture signal line driving circuit, which are integrally formed on the transparent insulating substrate, include a digital circuit which includes thin-film transistors of a polysilicon as basic elements and which includes one stage of CMOS buffer comprising an N-type thin-film transistor and a P-type thin-film transistor formed on the same substrate or a plurality of CMOS buffers connected in multi stages. In these CMOS buffers, for example, a CMOS transistor is connected so as to serve as an inverter. In addition, a pulse voltage having a duty ratio of about one-tenths through about one-thousandths is applied as an input signals, and the output signal thereof is applied to the scanning lines and the picture signal lines.
In the above described driving circuit including active matrix type liquid crystal display, it is known that the thin-film transistors serving as the basic elements of the driving circuit integrally formed on the transparent substrate are inferior to transistors using a mono-crystalline silicon substrates in capability. In order to explain an example thereof, FIG. 8 shows the relationship between a gate voltage Vg and a drain current Id. As shown in this figure, when the gate voltage Vg is 0V, the variation in the drain current Id caused by a slight difference in a producing process is great as shown by a width xcex94. In order to compensate such characteristics, it is required to increase a gate width to facilitate the flow of currents. Therefore, there is a problem in that the electric power consumption of each element increases in accordance with the increase of leak currents if the gate width is increased.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a driving circuit including active matrix type capable of decreasing the leak currents of thin-film transistors forming CMOS buffers, which are included in a scanning line driving circuit and/or a picture signal line driving circuit, to reduce electric power consumption.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a liquid crystal display having active matrix type liquid crystal display elements comprising switching elements connected to a plurality of scanning lines and a plurality of picture signal lines perpendicular to the scanning lines, the liquid crystal display including a scanning line driving circuit and a picture signal line driving circuit, the scanning line driving circuit applying a scanning pulse to said switching elements via the scanning lines and the picture signal line driving circuit applying a picture signal to the picture signal lines, at least one of the scanning line driving circuit and the picture signal line driving circuit comprising a digital circuit, the digital circuit comprising one stage of CMOS buffer or a plurality of CMOS buffers connected in multi stages, the CMOS buffer or each of the CMOS buffers including an N-type thin-film transistor and P-type thin-film transistor which are formed on the same substrate, wherein one transistor, which has a longer off-state time during operation of the circuit, of the N-type thin-film transistor and P-type thin-film transistor constituting the CMOS buffer, has a longer gate length than that of the other transistor.
According to another aspect of the present invention, there is provided a liquid crystal display having active matrix type liquid crystal display elements comprising switching elements connected to a plurality of scanning lines and a plurality of picture signal lines perpendicular to the scanning lines, the liquid crystal display including a scanning line driving circuit and a picture signal line driving circuit, the scanning line driving circuit applying a scanning pulse to the switching elements via the scanning lines and the picture signal line driving circuit applying a picture signal to the picture signal lines, at least one of the scanning line driving circuit and the picture signal line driving circuit comprising a digital circuit, the digital circuit comprising one stage of CMOS buffer or a plurality of CMOS buffers connected in multi stages, the CMOS buffer or each of the CMOS buffers including an N-type thin-film transistor and P-type thin-film transistor which are formed on the same substrate, wherein one transistor, which has a longer off-state time during operation of the circuit, of the N-type thin-film transistor and P-type thin-film transistor constituting the CMOS buffer, has a narrower gate width than that of the other transistor.
According to a further aspect of the present invention, there is provided a liquid crystal display having active matrix type liquid crystal display elements comprising switching elements connected to a plurality of scanning lines and a plurality of picture signal lines perpendicular to the scanning lines, the liquid crystal display including a scanning line driving circuit and a picture signal line driving circuit, the scanning line driving circuit applying a scanning pulse to the switching elements via the scanning lines and the picture signal line driving circuit applying a picture signal to the picture signal lines, at least one of the scanning line driving circuit and the picture signal line driving circuit comprising a digital circuit, the digital circuit comprising one stage of CMOS buffer or a plurality of CMOS buffers connected in multi stages, the CMOS buffer or each of the CMOS buffers including an N-type thin-film transistor and P-type thin-film transistor which are formed on the same substrate, wherein one transistor, which has a longer off-state time during operation of the circuit, of the N-type thin-film transistor and P-type thin-film transistor constituting the CMOS buffer, has a longer gate length than that of the other transistor, and the one transistor has a narrower gate width than that of the other transistor.